There are a range of airflow measurements for which an anemometer is the preferred solution. These include spot measurements of airflow are variable locations, such as beneath ceiling-mounted air-conditioning vents, for example. At such locations, it is aesthetically undesirable and technically unnecessary to provide a fixed or otherwise permanent airflow sensor, so a technician will take a handheld sensor to the location and hold it in the airflow of the vent
Such sensors typically comprise a rotateable vane that is mounted within a circular protective ring. The vane is typically of a metallic or plastics material, and a Hall effect or optical sensor in a handle portion extending from the ring can detect when one of the blades of the vane passes by. From this, the rotational speed of the vane can be determined, and knowledge of the aerodynamic properties of the vane will allow this to be converted to an airflow speed in the vicinity of the sensor.
Such anemometers suffer from distinct difficulties in practice. Principally, the rotating vane will have an inertia which must be overcome. This will impose a reaction time on the sensor output, and will make the sensor insensitive to small airflows. This can be reduced by reducing the mass of the vane, for example by using thin gauge sheet of a lightweight material such as aluminum, but such measures will reduce the rigidity of the vane and make it vulnerable to deformation on rough handling or shock, for example. Such deformation will change the aerodynamic properties of the vane and affect the accuracy of the sensor.
Further, the rotating vane is a moving part and hence in principle more vulnerable to wear, degradation, and the like.
Pitot-static devices are also used for measurement of airflow, as (for example) disclosed in GB-A-2,164,159. These are however bulky and have not been used for “on-the-spot” measurement via a handheld device.
A product known as the “Wilson Flow Grid” allows the measurement of airflow in a conduit such as a heating, ventilation or air-conditioning conduit or duct. It comprises a pair of square or circular grids of hollow conduits transverse to the airflow, one in front of the other. The frontmost grid has apertures in the sides of the conduits, facing into the airflow; these allow the dynamic pressure to be sampled. The rearmost grid has apertures on the two lateral sides of the conduit, at approximately 90° to the airflow, to sample the static pressure. In a rectangular grid (for a rectangular section conduit), the conduits form a gridiron pattern. In a circular grid (for a circular section conduit), the conduits are arranged as spokes from a central hub.
U.S. Pat. No. 4,453,419 shows a flow measurement device for use in a conduit, with two sets of radially-extending hollow spokes, one in front of the other. The spokes have apertures on their outer faces; thus one set has apertures facing forwards and one has apertures facing rearwardly. Each set of spokes emanates from one of two central hubs, from which dynamic and static pressure measurement are taken.
These devices are only suitable for use in fixed ducts or conduits, however. They are bulky and heavy, and not suited to portable use.